With a conventional electronic sphygmomanometer, an arm belt (cuff) is wrapped around a measurement site, and during a process in which the cuff (cuff pressure) is inflated to a higher pressure than systolic blood pressure and thereafter gradually deflated, the pulse produced by an artery is detected with a pressure sensor via the cuff, and systolic blood pressure and diastolic blood pressure are determined using the cuff pressure and the magnitude (pulse wave amplitude) of the pulse at that time (oscillometric method).
In contrast, a volume compensation type sphygmomanometer configured to continuously measure blood pressure per heartbeat in a non-invasive manner has been developed (see JP 54-50175A (hereinafter “Patent Literature 1”)). The volume compensation method is a method in which compression pressure (cuff pressure) is equalized with intra-arterial pressure, that is, blood pressure, by compressing an artery with a cuff from outside the body and keeping the volume of the pulsating artery uniform per unit length, and continuous blood pressure values are obtained by detecting the cuff pressure when this state is maintained.
By keeping the intra-arterial pressure and the cuff pressure on the artery constantly in equilibrium, the arterial wall is unloaded (i.e., natural state in which pressure is not exerted). In view of this, detecting the volume value (control target value) when the artery is in an unloaded state and maintaining this state (servo control) are two important points. In particular, because the accuracy of the control target value greatly influences blood pressure measurement accuracy, determining the control target value is very important.
A method of determining the control target value has been invented that involves detecting the maximum point of an arterial volume change signal (AC component of volume pulse wave) obtained from a photoelectric volume pulse wave or an impedance pulse wave while gradually compressing an artery using a cuff, and taking an arterial volume value (DC component of volume pulse wave) at that time as the control target value (see JP 59-156325A (hereinafter “Patent Literature 2”)). With an electronic sphygmomanometer using the volume compensation method of Patent Literature 2, a fixed control target value is used for the entire range of control cuff pressures.
However, the change in the DC component of the volume pulse wave under the influence of the deformation of body tissue around an artery in the process of compressing the artery is in actual fact greater than the change in arterial volume. Thus, the blood pressure may only be able to be measured on a finger where the deformation of body tissue is limited, and the measured difference (pulse pressure) between systolic blood pressure and diastolic blood pressure may be smaller than the actual blood pressure value.
A method of determining a control target value that incorporates the influence of the deformation of body tissue has been invented (see JP 8-581A (hereinafter “Patent Literature 3”)). In this invention, diastolic blood pressure and systolic blood pressure are calculated using an oscillometric method during the process of gradually compressing an artery, and the arterial volume values at these two points are detected. These two points serve as the control target values for diastolic blood pressure and systolic blood pressure. Interpolating these two points with an arbitrary curve enables control target values that incorporate the influence of the deformation of body tissue at arbitrary points within one heartbeat to be determined. Using control target values determined in this way enables measurement error that arises in the case where a fixed control target value is used to be eliminated.
Patent Literature 1: JP 54-50175A
Patent Literature 2: JP 59-156325A
Patent Literature 3: JP 8-581A